Fiber resin martices comprised of curable epoxy resin compositions and fiber reinforcement have found extensive use in applications where high strength, corrosion resistance and light weight are desired. Considerable effort has been expended in improving the properties and performance of such fiber resin matrix compositions, including the development of many different curing systems.
Amine and polyamine curing agents, such as m-phenylenediamine, 4,4'-diaminodiphenyl methane and diamino diphenyl sulfone (DDS), have become widely accepted. However, their characteristic toxicity, low solubility, high exotherm and variable curing rate has made further improvement in curing systems desirable.
In U.K. Pat. No. 1,182,377, a number of aromatic polyamines falling within the broad formula: ##STR1## wherein R is the radical formed after elimination of the hydroxyl groups of a polyhydric aliphatic, cycloaliphatic or araliphatic alcohol, Ar is an optionally substituted phenylene or naphthylene radical, R' is hydrogen or alkyl and n is an integer of 2 to 10, have been investigated as curing agents. They are reported to be less toxic and to provide less cracking when used as curing agents in epoxy compositions for glass-reinforced fiber resin matrices. It is also known from Moulton et al, U.S. Pat. No. 4,427,802, that aromatic polyamines can be modified prior to use by reaction with carbonyl compounds to avoid the drawbacks associated with all polyamines. Finally, Asahi, Japan 52-74665 (1977), teaches that if polyamine curing agents are used with tetrafunctional epoxy resins, carbon fiber-reinforced composites having excellent interlaminar strength and heat resistance will be provided.
From such disclosures it would be expected that all of the polyamine compounds described in U.K. Pat. No. 1,182,377 (formula (I), above) would give beneficial results in carbon fiber-reinforced composites. In fact, however, such expected benefits have not been found, and, when some of the compounds exemplified in U.K. Pat. No. 1,182,377 are substituted for the polyamine curing agents in the matrix resins of the Moulten et al or the Asahi patent compositions, gross deficiencies in physical properties occur, as will be demonstrated hereinafter.
It has now been discovered that the role of the bridging group designated R in compounds of the formula (I) above, is surprisingly more important in determining the ultimate physical properties and performance-in-use of the final fiber resin matrix composite than has been realized before by those skilled in this art. Applicants herein have discovered that the structural nature of the R radical has a profound effect on such properties, e.g., as glass transition temperature, flexibility and toughness in cured carbon fiber-reinforced epoxy resin composites. Moreover, it appears in some cases that these disparate effects are unique to non-siliceous fiber-reinforced resin matrixes and cannot be predicted from testing cured neat resin compositions or glass-reinforced resin composites. As will be demonstrated in comparative examples herein, the degree of branching, the chain length, and the steric bulk of the bridging R radical are all factors in the performance of the final fiber resin matrix composite. For example, a high degree of branching, such as where R is dialkylpropylene, tends to lower the glass transition temperature, flexibility and toughness of the fiber resin matrix; the length of alkyl or alkoxy chains in the R radical also affects properties, longer chains tending to lower the cross-link density of the cured resin, resulting in a lowered glass transition temperature; and the presence of bulky groups such as phenylene radicals also appears to lower rotational freedom or contribute steric bulk to the compound, and this leads to an undesirable decrease in the glass transition temperature.
It is a key feature of the present invention to judiciously select aromatic diamine curing agents for curable epoxy resin compositions to provide superior physical properties and performance in use after curing. Such selection will necessarily omit the compounds of the working examples of the U.K. Pat. No. 1,182,377, many of which are functionally characterized by bridging R radicals which adversely affect the properties of non-siliceous fiber-reinforced epoxy resin composites. The reinforced compositions prepared according to this invention show improved interlaminar toughness and residual compression strength, while maintaining compression strength under hot/wet conditions.
The present invention is directed to heat-curable epoxy resin compositions having non-siliceous fiber reinforcement and including a selected diamine curing agent of the general formula: ##STR2## wherein R is hydrogen or methyl and X is a divalent non-aromatic organic hydrocarbon, non-aromatic hetero-interrupted hydrocarbon, or non-aromatic halo- or alkyl-substituted hydrocarbon radical. In the preferred curable resin compositions of this invention, divalent X radicals will be selected to minimize branching, chain length and steric bulk, and thereby maintain excellent performance under hot and hot/wet conditions, e.g., high glass transition temperature (e.g., of 100.degree. C. or above) and compression strength. The superior performance of such compositions is shown in many of the working examples that follow.